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Structure of a Leaf
The function of a leaf is photosynthesis which is to absorb light and carbon dioxide to produce carbohydrates. The leaf has the following tissues:
1. Upper Epidermis:
this is the tissue on the upper surface of the leaf. It produces a waxy layer, called the cuticle, which is a waterproof barrier to prevent excessive evaporation through the hot upper surface of the leaf. The upper epidermis cells have no chloroplasts so light passes through them easily.
2. Palisade Mesophyll:
this tissue is where 80% of the photosynthesis takes place in the leaf. The palisade cells have many chloroplasts in their cytoplasm and the box-like shape and arrangement of these cells ensures they are packed tightly together.
3. Spongy Mesophyll:
this tissue contains large air spaces which are linked to the atmosphere outside the leaf through microscopic pores called stomata on the lower surface. Spongy mesophyll cells also contain chloroplasts and photosynthesis occurs here too. The air spaces reduce the distance carbon dioxide has to diffuse to get into the mesophyll cells.
4. Vascular Bundles:
made up of xylem tissue which carries water to the leaf and phloem tissue which carries the food away.
5. Lower Epidermis:
It contains specialised cells called guard cells which enclose a pore called a stoma. Carbon dioxide can diffuse into the leaf through the stomata when they are open (usually at day time) and water evaporates out of the stomata in a process called transpiration.
Watch the following video and then try to draw a leaf structure and label all the parts without referring to the text.
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